1. Field of the Invention
The present invention relates to the manufacture of electronic modules, and in particular, to methods of attaching electronic components to each other using Pb-free solder interconnections in combination with Sn/Pb or Pb-containing paste to provide a soldered article with acceptable and reliable levels of thermo-mechanical fatigue.
2. Description of Related Art
The use of solder to join materials such as components of an electronic structure is well known in the art. In the electronics area there are a myriad of electronic components that require connection to other electronic components or to other levels of packaging. Examples include mounting of integrated circuit chips to a metallized substrate, multi-layer ceramic substrate (MLC), laminate organic substrate, glass ceramic substrate, card (direct-chip-attach, DCA), printed circuit board (PCB) and any substrate made of composite materials meeting thermal and mechanical properties.
Conventionally, Sn—Pb type solder interconnections, having low tensile modulus, are used to join electronic components, such as substrates to electronic PCBs or cards. However, since these Sn—Pb type solder interconnections contain poisonous Pb, there has been an increasing number of cases in which the use thereof is restricted. To address these problems, trends have been leaning towards the use of interconnections composed of Pb-free solders for joining substrates to electronic PCBs. Several Pb-free solders have been identified for replacing Pb-containing solder interconnections in microelectronic applications, some of which include Sn-3.5Ag (SA), Sn-3.5Ag-0.7Cu (SAC), Sn-3.5Ag-4.8Bi (SAB), and Sn-0.7Cu (SC) (with slight variations in compositions).
As the interconnect industry diverges away from the use of Sn—Pb type solder interconnections, and towards Pb-free interconnections, there remains a period during which the use of a Sn—Pb type solder paste (or flux) will still be required for joining the Pb-free interconnections to the electronic PCBs due to the materials thereof. However, attempts at attaining a sufficient and acceptable level of thermo-mechanical fatigue reliability for these hybrid or mixed assemblies has not generally been successful.
Accordingly, during this interim period, there exists a need in the art for providing improved methods for connecting Pb-free interconnections to electronic PCBs using a Sn—Pb type solder paste (or flux) for forming a soldered article having superior reliability in the mixed (hybrid) assembly.